Effect of electronic asymmetry and the polymethine chain length on photoprocesses in merocyanine dyes

• Merocyanines with different electronic asymmetry were studied by ns-laser photolysis.
• Lifetime and spectral shift regularities of merocyanines photoisomers are revealed.
• DFT calculation of merocyanines structure and rotation barriers was carried out.
• Polymethine chain CC bonds involved into photoisomerization are determined.
• T–T-absorption and photostability of merocyanines were discussed.

The photophysical properties as well as photoisomerization of a series of malononitrile based merocyanines with various electron donating groups and polymethine chain length were studied by ns-laser photolysis. The absorption spectra and the kinetics of cis–trans backward conversion of the photoisomers were investigated in air-saturated solutions upon direct photoexcitation. The T–T absorption of merocyanines was recorded upon sensitized excitation with benzophenone or anthracene as the triplet energy donor and kinetics of the triplet state decay was measured. The obtained results together with the data of DFT/B3LYP quantum-chemical calculation allowed revealing the influence of the merocyanines electronic structure on the spectral properties and the lifetimes of the photoisomers. The long-wavelength absorption bands of photoisomers of the dyes with 3H-indole and benzimidazole electron donating groups were shifted bathochromically whereas the bands of the benzo[cd]indole derivatives were shifted hypsochromically with respect to the corresponding bands of the initial all-trans isomers. The interpretation of the results obtained is given.

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